1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a multi-domain liquid crystal display device in which a common auxiliary electrode is formed around and in a pixel region on a same layer as a gate line, and at least one or more electric field induction windows and dielectric structures are formed in the pixel region.
2. Discussion of the Related Art
Among flat-panel displays enjoying image quality equivalent to the image quality offered by a cathode ray tube (CRT) display, it is a liquid crystal display (LCD) that has been most widely adopted nowadays. In particular, a thin-film transistor (TFT) type LCD (TFT-LCD) has been adapted to widely used equipment such as personal computers, word processors, office automation equipment, and home electrical appliances, including portable television sets. The market for such equipment, using TFT-LCDs, is expected to expand. Accordingly, there is demand for further improvement in image quality. A description will be made by using a TFT-LCD as an example. However, the present invention is not limited to a TFT-LCD, but can apply to a simple matrix LCD, a plasma addressing type LCD, and so forth. Generally, the present invention is applicable to LCDs which include liquid crystal sandwiched between a pair of substrates on which electrodes are respectively formed and produce image displays by applying voltage between the electrodes.
Currently, a mode most widely adopted for the TFT-LCD is a normally-white mode that is implemented in a twisted nematic (TN) LCD. The technology of manufacturing the TN TFT-LCD has advanced extraordinarily in recent years. Contrast and color reproducibility provided by the TN TFT-LCD have surpassed those offered by the CRT. However, the TN LCD has a critical drawback of a narrow viewing angle range. This poses a problem that the application of the TN LCD is limited. FIGS. 1A to 1C are diagrams for explaining this problem.
In the Figures, reference numerals 11 and 12 indicate substrates and reference numberal 14 indicates liquid crystal. FIG. 1A shows a state of white display to which no voltage is applied and liquid crystal molecules are aligned in the same direction with a slight inclination(about 1° to 5°). For convenience, throughout the figures, liquid crystal molecules are illustrated as in FIG. 1A. In this white display state, light is seen as nearly white in any azimuth. Moreover, as shown in FIG. 1C, in the state in which a voltage is applied, intermediate liquid crystal molecules except those located near the alignment films (not shown), over the substrates, are aligned in a vertical direction. Incident linearly-polarized light is therefore seen as black but not twisted. At this time, light obliquely incident on an LCD screen (panel) has a direction of polarization that is twisted to some extent, because the light passes obliquely through the liquid crystal molecules that are aligned in the vertical direction. The light is therefore seen as halftone (gray) but not perfect black. As shown in FIG. 1B, in the state in which an intermediate voltage lower than the voltage applied in the state shown in FIG. 1C is applied, the liquid crystal molecules near the alignment films are aligned in a horizontal direction but the liquid crystal molecules in the middle parts of cells erect themselves halfway. The birefringent property of the liquid crystal is lost to some extent. This causes transmittance to deteriorate and brings about halftone (gray) display. However, this effect occurs only for light incident perpendicularly on the liquid-crystal panel. Obliquely incident light is seen differently, that is, light is seen differently depending on whether it is viewed from the left or right side of the drawing. As illustrated, the liquid crystal molecules are aligned mutually parallel relative to light propagating from right below to left above. The liquid crystal hardly exerts a birefringent effect. Therefore, when the panel is viewed from left, it appears black. By contrast, the liquid crystal molecules are aligned vertically relative to light propagating from below to right above. The liquid crystal exerts a great birefringent effect relative to incident light, and the incident light is twisted. This results in nearly white display. Thus, the most critical drawback of the TN LCD is that the display state varies depending on the viewing angle.
It is known that viewing angle performance of a liquid crystal display device (LCD) in the TN mode can be improved by setting the orientation directions of the liquid crystal molecules inside pixels to a plurality of mutually different directions. Generally, the orientation direction of the liquid crystal molecules (pre-tilt angles) is restricted by the direction of a rubbing treatment applied to the alignment film on the surfaces of the substrates as the liquid crystal molecule contact the alignment film. The rubbing treatment is a process, during which the surface of the alignment film is rubbed in one direction by a cloth such as rayon. The liquid crystal molecules are orientated in the rubbing direction. Therefore, viewing angle performance can be improved by making the rubbing direction different inside the pixels.
FIGS. 2A to 2C show a method of making the rubbing direction different inside the pixels. As shown in this drawing, an alignment film 22 is formed on a glass substrate 16 (whose electrodes, etc., are omitted from the drawing). This alignment film 22 is then bought into contact with a rotating rubbing roll 201, which rotates in a first direction, to perform the rubbing treatment in one direction. Next, a photo-resist is applied to the alignment film 22, and a predetermined pattern is exposed and developed by photolithography. As a result, a layer 202 of the photo-resist, which is patterned, is formed as shown in FIG. 2B. Next, the alignment film 22 is brought into contact with a rubbing roll 201, which rotates in a second direction opposite to the first direction so that only the open portions of the pattern are rubbed. In this way, a plurality of regions that are subjected to the rubbing treatment in different directions are formed within the pixel, and the multiple orientation directions of the liquid crystal are formed in the pixel. Incidentally, the rubbing treatment can be done in arbitrarily different directions when the alignment film 22 is rotated relative to the rubbing roll 201.
In the process described above, there are some problems creating boundaries for the different orientation directions of the liquid crystal molecules for improving the viewing angle performance in a vertical alignment (VA) LCD.
It is desirable to improve a viewing angle characteristic of a VA liquid crystal, display, and to create a VA liquid crystal display exhibiting a viewing angle characteristic that is as good or better than the one exhibited by in-plane switching mode LCDs, while permitting the same contrast and operating speed as the conventional liquid crystal displays.
In the VA mode employing a conventional vertical alignment film and using a negative liquid crystal as a liquid crystal material, a domain regulating means is included for regulating the orientation of a liquid crystal in which liquid crystal molecules are aligned obliquely when a voltage is applied so that the orientation will include a plurality of directions within each pixel. The domain regulating means is provided on at least one of the substrates. Further, at least one of domain regulating means has inclined surfaces (slopes). The inclined surfaces include surfaces which are almost vertical to the substrates. Rubbing need not be performed on the vertical alignment film.
In the VA-LCD device, when no voltage is applied, in almost all regions of the liquid crystal other than the protrusions, liquid crystal molecules are aligned nearly vertical to the surfaces of the substrates. The liquid crystal molecules near the inclined surfaces also orient vertically to the inclined surfaces, therefore, the liquid crystal molecules are inclined. When a voltage is applied, the liquid crystal molecules tilt according to electric field strength. Since the electric fields are vertical to the substrates, when a direction of tilt is not defined by a rubbing process, the azimuth in which the liquid crystal molecules tilt due to the electric fields includes all directions of 360°. If there are pre-tilted liquid crystal molecules, surrounding liquid crystal molecules are tilted in the directions of the pre-tilted liquid crystal molecules. Even when rubbing is not carried out, the directions in which the liquid crystal molecules lying in gaps between the protrusions can be restricted to the azimuths of the liquid crystal molecules in contact with the surfaces of the protrusions. When voltage is increased, the negative liquid crystal molecules are tilted in directions vertical to the electric fields.
Recently, a liquid crystal display device which drives a liquid crystal by an auxiliary electrode electrically insulated from a pixel electrode, without aligning the liquid crystal, has been suggested. Such a related art liquid crystal display device will be described with reference to FIG. 3.
As shown in FIG. 3, the related art liquid crystal display device includes a first substrate, a second substrate, a plurality of data lines 132 and gate lines 131b, a thin film transistor 134, and a pixel electrode 133. The data lines 132 and gate lines 131b are formed on the first substrate lengthwise and crosswise to divide the first substrate into a plurality of pixel regions. A dielectric projection 120a is formed on the pixel electrode in a zig-zag form, and a dielectric projection 120b is formed on the color filter layer in a formation similar to the dielectric projection 120a and is parallel to the dielectric projection 120a. Also, a light-shielding layer 135 is formed on a bending portion or a corner portion of the gate lines, the data lines, the thin film transistor and the dielectric projections 120a and 120b, so that light leaked therefrom is blocked.
The dielectric projections 120a and 120b divide the pixel region, and induce and distort the electric field applied to the liquid crystal layer. This means that dielectric energy due to the distorted electric field orients a liquid crystal director in a desired direction when a voltage is applied to the liquid crystal display device.
However, the liquid crystal display device has several problems. The dielectric projections 120a and 120b can obtain multi-domain effect, but reduce aperture ratio. To solve this problem, the dielectric projections are formed with narrow widths. However, the thick light-shielding layer 135 formed to prevent shadow from being generated at the bending or corner portion of the dielectric projections still causes problems related to aperture ratio.
Moreover, if the dielectric projections are not formed or if they have quite small widths, the distortion range of the electric field required to divide the domain is weak. Accordingly, there is a problem that the time it takes to orient the liquid crystal and to reach a stable state is increased.